The present invention relates to digitized speech communications systems and more particularly to a conferencing circuit for use in a time division multiplex loop communications system.
In a digitized voice, time division multiplex communications system, all of the subscriber stations are connected on a common communications path which is allocated to each subscriber station on a time division basis. Analog voice signals input at each of the subscriber stations are sampled at a constant rate and each sample is converted into an equivalent digital word. During designated time slots each subscriber station removes from the loop the digitized speech sample sent to the subscriber station while during another time slot there is placed on the loop the output speech sample from the subscriber station. The speech samples are provided to each subscriber station and returned from each subscriber station at such a rate as to make possible continuous communication between the subscriber station despite the sharing of the communications path.
A conferencing circuit in a communications system enables a plurality of subscriber stations to engage in a group conversation. Numerous circuits of this type have been developed, both for analog and digital applications. With the development of digitized voice communications using time division multiplexing, a need has arisen for a digital conferencing circuit which can group any number of subscriber stations, does not interfere with non-conference communications, and effectively suppresses side-tone interference.
A prior art digital conferencing circuit is shown in U.S. Pat. No. 3,551,600 to Berch. In this circuit, a number of telephone lines are scanned, the voltage digitized and the resulting digital words summed for all the lines. The signal returned to each subscriber station has that particular station's input removed through algebraic subtraction. Although this circuit does provide side-tone suppression, it does not include means for passing through non-conference conversations and does not provide for a digital selection of conferees.